In many applications, in office buildings, multiple residential buildings, small industrial complexes, and others, fusible switches and circuit breakers are directly competitive. In this competitive situation, fusible switches have the advantage of lower cost in comparison with circuit breakers. On the other hand, a circuit breaker usually affords a more positive indication of operation in response to an overload and also is more compact in size. Due to the size differential, fusible switches and circuit breakers have not been truly interchangeable; that is, for most applications it has not been possible to make a direct substitution of fusible switches for circuit breakers or vice versa.
Common service entrance voltage ratings are 240 volts and 600 volts. Depending upon the circuit served, the current rating for a fusible switch or circuit breaker may be 30 amperes, 60 amperes, 100 amperes, or 200 amperes. Considering all of these possible voltage/current ratings, the cost of components for individualized fusible switches, using different components for each switch rating, can be a critical factor in the competitive equation.
The cost of fusible switches usable over a broad range of voltage and current ratings could be materially reduced by use of essentially identical, interchangeable component parts in switches having substantially different ratings. In this way, tooling and manufacturing costs can be materially reduced. But the use of identical parts for switches of different ratings creates a substantial danger of mistakes in assembly of the switches or in installation and use of those switches. Thus, if a 600 volt fusible switch, as assembled, permits installation of 240 volt fuses, a dangerous situation may be created, even though the incorrect fuse has the correct current rating. Similarly, if a 240 volt fusible switch of the pull out type allows insertion of a 600 volt head or if a 100 ampere switch head can be inserted into a 60 ampere or 30 ampere switch base, an obviously dangerous condition may be created. Consequently, standardization of individual components for use in fusible switches of different voltage and current ratings has been relatively limited.